Sound recording



I July 21,1936. I O M, GLUNT 2,048,500

SOUND RECORDING Filed Oct. 1, 1950 2 Sheets-Sheet 1 INVENTOR 0. M. GL UNT I By I A T TORNE) July 21, 1936. o. M. GLUNT 2,048,500

SOUND RECORDING Filed 001:. l, 1930 2 Sheets-Sheet 2 N Q 3 k l" m aw a 9l 1 INVENTOR, O.M.GLUN7 \H q Hu dt ATTORNEY Patented July 21, 1936UNITED STATES- SOUND RECORDING Omer M. Glunt, Rutherford, N. J.,assignor to Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application October 1, 1930,Serial No. 485,585 7 In Great Britain September 4,1 930 3 Claims.

This invention relatesto sound recording and particularly to a method ofrecording by photographic means a wider range ofsound volumes than canbe recorded by known methods.

The object of this invention is to reduce the volume of noise producedfrom" a photographic sound record during the reproduction of the soundfrom the record. 7 A reduction in the volume of noise produced from therecord will permit smaller volumes of sound to be satisfactorilydistinguished from the noise thus increasing the range of volumes ofsound which may be recorded. Or, if the increased range is not utilized,the invention will provide a larger ratio between the present minimumvolume of sound and the volume of noise produced during the reproductionof the sound and will thus produce a more pleasing result.

A feature of the invention is a method of re.- cording sound byphotographic means in which for a given frequency the time of exposureof the negative sound record is constant, but the intensity of therecording light varies directly with the envelope of the volumevariations of the sound recorded and the light incident on the film ismodulated to a constant degree for all volumes of sound.

Another feature oi -the invention is a method of producing a negativephotographic sound record in whichthe average opacity of the soundrecord varies directly with the envelope of the volume variations of thesound and the average opacity for any given volume is modulated to aconstant degree.

Afurther feature of the invention is a deyice ree sponsiveto soundmodulated currents which varies the amplification of the recordingamplifier inversely in accordance with the envelope of the amplitudevariations ofthe sound modulated currents thus maintaining" the outputpower of the amplifier substantially constant.

In any known method of recording sound the range of volumes of soundwhich may be recorded satisfactorily will be limited by the propertiesof the recording medium used and the handling which the completed recordreceives. The least volume of sound which is of interest in recording isthe volume of soundwhich in reproduction may be satisfactorilydistinguished above the noise produced by irregularities in thestructure of the mediums used in recording and reproducing and the dirt,scratches, etc. which collect on the record during its useful life.Recent experiments have shown that the volume of noise produced duringthe reproduction of a film soundjrecord is approximately proportional tothe average transmission of the positive sound record. The averagetransmission of the sound recordmay be considered tolbe equa m thetransmission of the unmodulated portions of the sound record., [Thetransmission of a photographic medium is defined as the ratio of thelight transmitted through the medium divided by the light incident onthe medium and for convenience is commonly expressed as a percentage ofthe incident light. The opacity of the mediumis defined as thereciprocal of the transmission. g

If the transmission of the positive sound record 7 I is at alltimes assmall ,as possible'the noise will then be a minimum. It 'may be shown,however, that the volume of the reproduced sound isVpro-' portional totheproduct'of the average transmission of the positive sound recordmultiplied by the percentage modulation of this averagetransmission dueto thejrecord of the sound. vThus, in order to avoid distortion of thevolume-of sound if the average transmissionoi the positive sound recordis reduced thepercentage modulation of this transmission must beinversely increased. Obviously if the percentage modulation is at-alltimes as large as possible and the volume of sound is not distorted theaverage transmission and the noise will be a minimum. 1 For convenienceof description the percentage modulation has been assumed to be'maintainedas nearly100.% as may be practical though any otherpercentage may be used if desired.

As a photographic positive is 'an inverse copy of the photographicnegative from which the positlve was printed,'in order for the positiveto have a small transmission, the negative must have a largetransmission or a small opacity. It is well known that the opacity of aphotographic negative is directly; proportional to the exposurewhen theexposure lies within certain limits, and thatthe exposure isproportional to the product of the intensity of the light incident onthe negative multiplied by the time during which the negative issubjected to the light.- ;Thus assuming that the photographic processesare controlled so that no distortion is produced, the transmission ofthe positive sound record will be proportional to the exposure of thenegative sound record. Accordingly, to produce a .positive soundrecordhaving a small average transmission theexposure of thenegative soundrecord must be cor'resp'ondi'ngly small. In accordance with theinvention the intensity of the recording light is reduced inaccordancewith the envelope of the volume of sound recorded to producea; negative sound record havwill be proportional to,

' be proportional to a Y ing a large average transmission or a. smallaverage opacity for small volumes of sound. a e

In the present systems of sound recording the gainof the amplifiersupplying the recording device is usually adjusted so that the loudestsound to be recorded will produce substantially complete' modulation oftherecordinglight. The intensity 'of the recording light is thenadjusted so that the exposure produced by the unmodulated light is 'inthe center of the allowable range of exposures for thephotographicemulsion g s ed. After the sound is recorded andthe'negative sound record developed, positive prints are made. from-thenegative sound record usinga printing light having the intensityadjusted-to produce an a average transmission of about 40% for the'posi'tive sound record. In accordancewith'the'present invention the gain ofthe amplifier supplying 7 energy to the recording device isautomatically varied in accordance with the envelope of the volumevariations of the sound being recorded to ,produce a constant.-percentage of modulation of' the ilightflincident onthefilm for allvolumes of sound recorded and the intensity of the recording" light isvari'ed'inversely to the variation in thegain of the recordingamplifier.

'Consider 'three volumes of sound 'such' that under. the present methodone volume'produc'esj 100% modulation of the light incident on the le'ative' filmgaj secondrvolurne produces {10%, 'modulation and athirdjvolume produces 1% mod- 7 s V ulation, and that the undesirednoise produced inf reproduction is equaltothe third'volume of 7 sound.Assume also that the printing light is'ad- 'justed so that theaveragetransmission of the unmodulated sound track ofth'e positive printis 40%. Let S=the light energy passing through the negative for anygiven volume of sound. Then the light energy passing through'thepositive will be140% ofthis value, that is p 40 e i V nt i themodulationof this energy-is assumed to be 100%,thatis in the first' case, heuseful sound reproduced V In the second case the modulationis'onlythusthe useful sound reproduced will be proportional to N V 7 Similarly,in the third case the useful sound will 'be proportional to i V 'ifi xiofi m moo r l and the noise producedin reproductionwillalso e 9 Thus thevolume of the noise "is equalto 1% of the volume of the mudeShuSem'sound, 10% of the volume of the second sound and is equal to the volumeof the faintest sound.

Now in accordance with this invention, let the 7 gain of the recordingamplifier be adjusted to produce substantially complete modulation ofthe recording light for all volumes of sound re corded and let theaverage intensity of the recording lightbe adjusted inversely totheeadjustment of the gain of the amplifier. Assume also that theprinting light is adjusted so that the average transmission of theunmodulated sound track of the -loudest sound on the positive print is40%.

For the loudest sound the useful sound reproduced will now beproportional as before to i In the second case the intensity of therecord- :ing light is reduced to of its original value. Thus the lightenergy passe ing through the positive is now I I 10o 1o 1000 v 7 whilethe modulation is 10 times its original value,'that' is r r e 7 1 1 Theuseful sound reproduced will now bepropor tional to V V 7 a In the thirdcase, the intensity of the recording: lamp is reduced to a .1

1 1007 I l V V of its original value and the modulationis increased tothus the useful sound reproduced is nowproportionalto V I rdt m nt weThe light energy passing through the positive in the third case is thelight energy efiective in iSX : causing noise and this energy is nowreduced to The modulation due to the noise is unchanged, however, as itis due to the structure of the film thus the noise produced in'reproduction is now proportional to e Thus the volume of the noiseproduced during re production is now' reduced to 1% of the volume of thefaintest sound; that is,'the ratiobetween the faintest sound andthenoise has'been increased, 7 one hundredfold, r

It will be appreciated that the numerical values used in the aboveexplanation are illustrative only of the order of the advantage whichmay be se-' cured by this invention anda're' not to be construed aslimiting this invention in any way to the exact values of the quantitiesassumed.

In the drawings:

Fig. 1 represents in schematic form a film sound recording systemembodying one form of theinvention.

Fig. 2 represents in schematic form a film sound recording systemembodying .another form of the invention.

Sound waves are detected by a known type of microphone 8, and theresulting sound modulated currents may be amplified, if desired, in asuitable amplifier known in the art. A part of the amplifier currentsare applied through a transformer l2 to the input circuit of anamplifying device l3. The output of the device I3 is applied through atransformer I4 to the input circuit of a second amplifying device I5.The output of the device I5 is applied through a transformer Hi to themovable elements I! of a light valve. It will be understood that more orless than two amplifying devices in cascade may be used, and that otherknown systems of coupling these devices may replace the transformercoupling shown.

For convenience of description, the light valve is shown as amodification of the light valve disclosed in U. S. Patent 1,638,555August 9, 1927 to E. C. Wente. An image of the filament of a suitablelamp I8 is formed by the lens system IS on the small slot defined by themovable elements I? in the orifice 28 pierced in the pole pieces of apermanent magnet 2|. Suitable pole faces concentrate the magnetic fieldof the magnet 2| to form a uniform magnetic field in which the movableelements H are immersed. The movable elements are mechanically mountedand spaced to define a small light transmitting slot in the orifice 20.An electric current flowing in the movable elements will cause theelements to be displaced by the reaction of the magnetic field due tothe current and the uniform field due to the magnet 2| and thus to varythe height of the slot. An image of the slot is formed by an opticalsystem 22 on a photographic film 23 moved in a known manner by thesprockets 24, 24 at uniform speed past the point of exposure. Anaperture plate 25 holds the film smooth'at the point of exposure andprevents diffused light from fogging the record formed on the film.

Plate current is supplied from a battery 26 through a coupling resistor21 to the amplifying device 55, and flows through the device to wire 28.Plate current is similarly supplied from the battery 26 through resistor29 to the device I3.

The combined plate currents flow through resistor 38 back to the battery26. A condenser 3| bridged across the resistor 30 prevents soundmodulated voltages from being developed between the ends of the resistor30. modulated currents are applied to the amplifier,

When no sound the bias voltage applied to the control elements of.theamplifying devices I3 and I5 is determined by the magnitude of thepotential drop in the resistor 3|] due to the combined plate currents.

Referring now more particularly to Fig. 1 a portion of the soundmodulated current is applied through a transformer 32' to a detector 33,which may be a thermionic triode. A battery 34 impresses on the controlelectrode of the triode a potential of such magnitude that the outputcurrent of the triode varies with the envelope of the amplitudevariations of the sound modulated currents. A condenser 35, shown indotted lines in Figs. 1 and 2 may be used to aid the detector action ofthe triode. Current from the battery 23 passes through the triode 33,wires 36 and 28, and resistor 33 to the battery 26. A potential drop isdeveloped across the resistor 30 by the pulsating current flowingthrough the detector 33,

and this potential drop varies the bias applied to the grids of theamplifying devices I3 and I5.

It is well known that when the output of a detector energized by soundmodulated currents varies the grid bias of an amplifier amplifying thesame currents, by a suitable choice of the constants of the circuit, thegain of the amplifier may be caused to vary inversely with the envelopeof the amplitude variations of the sound modulated currents and theoutput power of the amplifier maintained substantially constant for awide range of amplitudes. The amplitude of vibration of the movableelements will thus be approximately constant, and, by suitable design ofthe amplifier, the amplitude of vibration may be made of such magnitudeas to produce substantially complete modulation of the light incident onthe fi m 23 for all volumes of sound. The period of the vibration of themovable elements however will vary with the frequency of the soundmodulated currents. Thus the action of the light valve may be consideredto be the production of a latent image in the emulsion of the film whichis a pattern of the frequency variations of the sound reduced to aconstant amplitude. While it may be true that this latent image possiblyhas no physical existence, it is believed that the use of this idea isjustified by the aid it gives in obtaining a clear understanding of theprocesses involved.

A portion of the sound modulated current is applied through atransformer 31 to a detector 38, which may be a thermionic triodesimilar to detector 33. A battery 39 impresses on the control electrodeof the triode a potential of such magnitude that the output current ofthe triode is a pulsating current varying with the amplitude of thesound modulated currents. Current flows from the battery 4|] through thewinding 4| to the output circuit of the detector 38 and through thedetector 38 to the battery 40.

The filament of the lamp I8 is supplied with current from the armature42 of a generator which is driven by a suitable motor, not shown. Thegenerator is excited by a field winding 43 supplied with current from abattery 44 through a variable resistor 45. The current in the excitingfield 43 maybe adjusted so that, in the absence of sound modulatedcurrents, the voltage generated by the armature is of such value thatthe current flowing in the filament of the lamp I8 is large enough toheat the filament but not large enough to cause the filament to emitlight rays which will cause a record on the film 23. When soundmodulated currents are applied to the detector 38, a current varyingwith the amplitude of the'sound modulated currents flows in theregulating field winding 4| and increases the voltage generated by thearmature 42. An increased current thus flows in the filament of the lampl8 and causes the light emitted by the lamp to vary with the currentflowing in the winding 4|. Due to the electrical and magnetic inertia ofthe exchange of power and the thermal lag of the filament, the lightwill not vary with the instantaneous variations of the sound modulatedcurrents, but rather with the envelope of the amplitude variations ofthe sound modulated currents.

The light emitted by the filament of the lamp |8 will thus be small forsmall volumes of sound and will vary directly with the envelope of thevolume variations of the sound. Thus the action of the lamp l8 may beconsidered to be the production of a latent image in the emulsion of athe film which varies with, the envelope o f the "volume variations ofthe sound and which is thesound'to be recorded Thefaverage opacity.

of the impression in the medium will vary with the intensity of thelight; The light incident 7 on the film is simultaneously modulated bythe lightyvalve, The modulation ofthe light'issub- The time of exposureof the film is varied by the stantially complete for all volumes ofsound.

light valve in accordance with the frequency pattern of the sound. l a

' While a preferred formof control of the inten-- sity of the recordinglight has been disclosed, it will be-obvious .to those skilled in theart that ma ny. modifications of this control may be made.

For examplea lamp having a reasonably thin filament maybe usedand thefilament kept warm by an auxiliary battery. 'The rectified soundmodulated currents may then be applied directly to the filainentor asuitable amplifier. maybe used to' amplify the'rectifie'd currents tosuch having at least. one movable element vibrated in. the direction ofmotion of said medium by the value that the amplifiedcurrents willdirectl vary the intensity of the-light} In Fig 2 the circuit elementswhich have they same function'and operate in the same manner as thecorresponding elements in Fig. 1 have been correspondingly numbered. Asthe operation of these elements has been given in detail in thedescription of Fig. 1, only the elements ,in'Fig. 2

which operate in a new manner, need be described. r v I A portion of thesound modulated currents from the amplifier l l is applied through the Itransformer 31 to the input circuit of a detector 41. A battery 39maintains the control electrode of the detector'd'l at such a potentialthat the output current of the detector 41- is a pulsating'currentvarying with the amplitude of the sound modulated'currents. Current issupplied by the battery 26 to the output circuit of the detector, andflows through the detector 41,

tensity of the lamp, and the same currents flowingJin'the resistor 30will control the gain of theamplifier as described in detail underFig. 1. The single detector 41 combines thelfunctions of the twodetectors 33 and 38 and the system shown in Fig. 2 willin general operate' in the constant the amplitude of vibration of said ele-' latedcurrents.

same manner as the system shown in Fig. 1. In some cases, practicaladvantages are gained by the use of separate detectors and controlcircuits. The filaments of the detector's 33, 38 and 41 and the cathodeheaters of the amplifiers l3 "and I5 are supplied with suitablepower'in'a mannor well known in the art.

What is claimed is: v g V 1. In a system for recording sound ona'photographic, medium, in combination, a source of. 7

light, a light valve having at least one movable element vibrating inthe direction of "motion' of J said medium and controlling thetransmission of light from said sourceto the full effective width ofsaid medium, .a source of sound modulated currents vibrating saidmovable elements, means for controlling the power delivered to said'elements by said source to maintain substantially ments for all volumes.of sound and a control cirvary the intensity of said source of light inaccordance with the envelope of the sound modulated currents.

.2. In a system for recording sound on a photographic medium, incombination, a source of sound .modulated currents, an amplifiersupplied With said currents, a source of light, a light valve and acontrol circuit energized by sound'modulated currents to vary theintensity of said source of light in accordance with the envelopeo-fzsaid sound modulated currents.

I 3. In a system fo'rrecording sound on a photographic medium, incombination, a source of sound modulated currents, an amplifier suppliedwith said currents, a device responsive to said sound modulated currentsfor maintaining the output ofsaid amplifier substantially constant, anelectric lamp, a recording device energized by the output of saidamplifier; and producing substantially complete modulation of the lightfrom said lamp incident on said mediumfan electric generator supplyingenergy to said lamp and having a regulating winding, and a controlcircuit energizing said regulating winding in accordance with theenvelope'of said sound modu- OMER M. GLUNT.

' cuit energized by sound modulated currents to

